Method and apparatus for measuring the thickness of compressed objects

ABSTRACT

A method and apparatus for determining a degree of deflection in a breast compression plate. The mammography apparatus further includes an optical measuring device. The method and apparatus involve (a) providing a pattern on the breast compression plate, the pattern being imagable by the optical measuring device, and having a plurality of local pattern indicia; (b) adjusting the breast compression plate to a selected height; (c) imaging the breast compression plate using the optical measuring device to provide an image of the pattern, the image having a plurality of local image indicia including an associated local image indicia for each local pattern indicia in the plurality of local pattern indicia; and (d) for each local pattern indicia in the plurality of local pattern indicia, determining an associated local deflection of the breast compression plate from the associated local image indicia.

FIELD OF THE INVENTION

This invention relates in general to a method and apparatus formeasuring the thickness of compressed objects, and more specificallyrelates to a method and apparatus for determining the degree ofdeflection of a breast compression plate in a mammography apparatus.

BACKGROUND OF THE INVENTION

In conventional mammography, a woman places her breast on a breastsupport plate. A detector is typically mounted under the breast supportplate. This detector is sensitive to x-rays. A breast compressor platethat is transparent to light and x-rays presses against the top of thebreast to flatten it and to prevent any movement during the mammographyprocess. An x-ray source is then turned on to image the breast, which isbetween the breast support plate and the breast compression plate.

The transmitted x-ray intensity through the breast is dependent on boththe composition of the breast and its local thickness. Most currentmammography machines have thickness indicators, which are imprecise.These thickness indicators do not show true breast thickness if eitherthe breast support plate or the compression plate bend or flex. This, inturn, will affect the accuracy and precision of the images of the breastobtained.

Techniques for determining the degree of flex of the breast compressionplate have been devised. For example, see Burch, A. and Law, J., AMethod for Estimating Compressed Breast Thickness During Mammography. BrJ Radiol 68, (1995) 394-399, which discloses using the magnification oflead markers placed on top of the compression plate. However, thismethod requires that all the markers be shown in the image, and does notmeasure the flexing of plastic plates.

Accurate measurement of compressed breast thickness is an importantfactor in determining volumetric breast density. It is also an importantfactor in determining dose calculation (for example, Wu. X., Gingold E.L. Barnes G. T., Tucker D m Normalized average glandular dose in Mo/Rhand Rh/Rh target-filter mammography radiology 1994 193 at 83 to 89; Law,J., editor: The Commissioning and Routine Testing of Mammographic X-raySystem, Institute of Physical Sciences in Medicine, York UK, (1994) 59;Dance, D. R.: Monte Carlo Calculation of Conversion Factors for theEstimation of Mean Glandular Breast Dose, Phy Med Biol, 35, (1990)1211-1219. The measurement of actual thickness provided by commercialmammography can be as much as one centimeter off the actual thicknessdue to deflection of the breast compression plate.

Accordingly, a mammography apparatus and method that improves theaccuracy of measuring breast thickness is desirable.

SUMMARY OF THE INVENTION

An object of an aspect of the present invention is to provide animproved method of determining a degree of deflection in a breastcompression plate.

In accordance with an aspect of the present invention, there is provideda method of determining a degree of deflection in a breast compressionplate of a mammography apparatus. The mammography apparatus furtherincludes an optical measuring device. The method comprises: (a)providing a pattern on the breast compression plate, the pattern beingimagable by the optical measuring device, and having a plurality oflocal pattern indicia; (b) adjusting the breast compression plate to aselected height; (c) imaging the breast compression plate using theoptical measuring device to provide an image of the pattern, the imagehaving a plurality of local image indicia including an associated localimage indicia for each local pattern indicia in the plurality of localpattern indicia; and (d) for each local pattern indicia in the pluralityof local pattern indicia, determining an associated local deflection ofthe breast compression plate from the associated local image indicia.

An object of a further aspect of the present invention is to provide animproved mammography apparatus for imaging breasts.

In accordance with a further aspect of the present invention, there isprovided an apparatus comprising (a) a breast compression plate forcompressing a breast to be imaged, the breast compression plate having avertical adjustment means for adjusting the height of the breastcompression plate to a selected height, and an optically-readablepattern; (b) a breast imaging means for imaging the breast compressed bythe compression plate; (c) an optical measurement means for generatingan image of the optically-readable pattern; and, (d) calculating meansfor determining a deflection of the breast compression plate from theselected height from the image of the optically readable pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of preferred aspects of the invention is providedherein below with reference to the following drawings, in which:

FIG. 1, in a perspective view, illustrates a mammography machine inaccordance with the preferred embodiment of the present invention;

FIG. 2 is a schematic drawing showing the optical axis of a CCD(Charge-Coupled Device) camera, and the orientation of this optical axisrelative to a horizontal plane having a grid-like pattern;

FIG. 3, in a schematic view, illustrates the optical pattern of one ofthese squares of the pattern of the horizontal plane of FIG. 2;

FIG. 4, in a graph, plots the pattern shift of different rows of thegrid pattern of FIG. 2 against thickness; and,

FIG. 5 in a three-dimensional graph, plots thickness in centimetersagainst grid column and row location.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Referring to FIG. 1, there is illustrated in a perspective view, amammography machine 12. The mammography machine 12 includes a breastsupport plate 14, a breast compression plate 18, an x-ray tube 16 and acamera 20. In operation, the x-ray tube 16 projects x-rays through thebreast compression plate 18, which is transparent to light and x-rays,through the breast, and through the breast support plate 14. The breastcompression plate 18 may be vertically adjusted to accommodate breastsof different dimensions. The breast support plate 14 includes a detector(not shown) that is sensitive to the x-rays. Variation in the density ofthe breast will have an effect on the x-rays traveling through thebreast, which will affect the image left on the detector in the breastsupport plate 14. These signal variations may then be examined forpossible tumors or other conditions. As discussed above, the transmittedx-ray intensity through the breast depends both on the composition ofthe breast and the thickness of the breast. Accordingly, to properlyinterpret the image, the thickness of the breast must be known. Toaccurately know the thickness of the breast, the deflection of thebreast compression plate 18 must also be known. Deflection of the breastsupport plate 14 is much less of a concern, as this plate iscomparatively rigid.

According to an embodiment of the present invention, a grid-like patternis provided on the top of the breast compression plate 18. Referring toFIG. 2, the orientation of this camera 20 is shown relative to agrid-like pattern 22 on the upper side of breast compression plate 18.The camera 20 has an optical axis 24 that forms an angle α relative tothe vertical axis 26 normal to the horizontal plane of the breastcompression plate 18 bearing the grid-like pattern 22. As will beapparent to those skilled in the art, when the grid-like pattern 22 ismoved up or down the vertical axis, this will change the image of eachintersection point 28 generated by the camera 20. This is illustrated inmore detail in FIG. 3 with respect to one square of the grid-likepattern 22, and specifically with respect to point 28 ₀.

Referring to FIG. 3, there is illustrated in a not-to-scale schematicview, how the image on the camera changes based on flexion or othervertical movement of the breast compression plate 18. The location ofthe lens of the camera 20 is marked 20 a in FIG. 3. A line of sight 30connects intersection point 28 ₀ with lens location 20 a. This line 30is then extended to an image plane 32. The intersection of the line 30with the image plane 32 marks the image point 34 ₀ corresponding tointersection point 28 ₀. The intersection point 28 ₀ is displaced fromthe optical axis by an initial distance A. The angle between line ofsight 30 and optical axis 24 is θ₂.

Say that the breast compression plate 18 is flexed upwardly by a breastcompressed between the breast compression plate 18 and the breastsupport plate 14. This upward displacement will vary from intersectionpoint 28 to other intersection points 28 based on fluctuation in breastthickness and compressibility. However, for the particular intersectionpoint 28 ₀ with which FIG. 3 is concerned, the deflection is by anamount T in a direction normal to the horizontal plane of the breastcompression plate 18. As a result of the deflection of intersectionpoint 28 ₀ to a new position shown as 28 _(d), a new line of sight 36 tothe lens position 20 a is generated. When extended back to the imageplane 32, this line of sight 36 intersects at new image point 34 _(d).This new image point 34 _(d) is a shift S from the initial image point34 ₀ and is displaced from the optical axis 27 by a distance S₀. Line ofsight 36 is at an angle θ₁ to optical axis 24.

As shown in the drawing, the lens position 20 a is defined by angle α,as well as by distances L and L₀, both of which distances are measuredparallel to the optical axis 24. Distance L is the distance from thecamera lens 20 a to the plane of the displacement point 28 ₀. L₀ is thedistance from the camera lens 20 a to the image plane 32. The parametersT₀, S₀ and θ₂ are fixed relative to each intersection point 28, althoughthese parameters change for different intersection points 28. Theparameters θ₁, S and T change depending on the degree of flexing of thebreast compression plate. However, as shown below, the thickness Tvaries linearly with the shift S.

From trigonometry, tan(θ₂)=A/L=(S₀−S)/L₀ andtan(θ₁)=[A+T₀*tan(α)]/(L−T₀)=S₀/L₀A/L=(S ₀ −S)/L ₀  (1)and[A+T ₀*tan(α)]/(L−T ₀)=S ₀ /L ₀  (2)Finally, based on trigonometry and then solving for T₀ using equations(1) and (2)T=T ₀/cos α=S(L/L ₀)[S ₀ cos α/L ₀+sin α]  (3)

These results were also verified empirically by changing the position ofthe breast compression plate. The shift of grid marks on the image platewas then mapped as a linear function of the height of the breastcompression plate 18 for each X, Y coordinate of the grid pattern.Referring to FIG. 4, the relative shift for intersection points 28 in afirst row 22 a, a third row 22 b and a fifth row 22 c of the gridpattern 22 are shown. Clearly, θ₂ increases with the horizontal distancefrom the camera such that for the fifth row 22 c θ₂ is larger than θ₂ isfor the first row 22 a. θ ₂ for the third row 22 b is smaller than θ₂for the fifth row 22 c, and is larger than θ₂ for the first row 22 a. Asa result, the shift S for intersection points 28 in the fifth row 22 cwill be greater than the shift S for intersection points 28 in the firstrow 22 a. However, in both cases there is a linear relation between theshift S and the thickness T. This is clearly reflected in the linesplotted in the graph of FIG. 4. Accordingly, the thickness at anindividual intersection point 28 can be computed from an image of thegrid pattern 22 taken along with each mammographic study.

Referring to FIG. 5, there is illustrated in a three-dimensional graph,a thickness map. The thickness map plots the displacement between thebreast compression plate 18 and the breast support plate 14 at differentX and Y positions on the grid. This displacement can be determined fromthe image of the grid at these X and Y positions in accordance with theabove-described method.

Other variations and modifications of the invention are possible. Forexample, other non-grid patterns may be provided on the top of thebreast compression plate to enable its deflection to be determined. Allsuch applications, modifications or variations are believed to be withinthe sphere and scope of the invention as defined by the claims appendedhereto.

1. A method of determining a degree of deflection in a breastcompression plate of a mammography apparatus, the mammography apparatusfurther including an optical measuring device, the method comprising:(a) providing a pattern on the breast compression plate, the patternbeing imagable by the optical measuring device, and having a pluralityof local pattern indicia; (b) adjusting the breast compression plate toa selected height; (c) imaging the breast compression plate using theoptical measuring device to provide an image of the pattern, the imagehaving a plurality of local image indicia including an associated localimage indicia for each local pattern indicia in the plurality of localpattern indicia; and (d) for each local pattern indicia in the pluralityof local pattern indicia, determining an associated local deflection ofthe breast compression plate from the associated local image indicia. 2.The method as defined in claim 1 wherein the breast support plate isadjustable, the method further comprising moving the breast supportplate to a plurality of different positions; and for each position inthe plurality of different positions, determining a position-specificimage of the pattern, the position-specific image including anassociated position-specific local image indicia for each local patternindicia at each position in the plurality of positions; wherein step (c)comprises, for each local pattern indicia in the plurality of localpattern indicia, determining the associated local deflection of thebreast support plate by correlating the associated degree of shift inthe associated local image indicia with an associated position-specificlocal image indicia.
 3. The method as defined in claim 1 wherein thepattern is a two-dimensional radio-lucent pattern.
 4. The method asdefined in claim 3 wherein the pattern is a grid, and each local patternindicia is a point on the grid.
 5. The method as defined in claim 2wherein the step of determining a position-specific image of the patternfor each position in the plurality of different positions, comprisesimaging the breast support plate at each different position using theoptical measuring device to provide the position-specific image of thepattern.
 6. A mammography apparatus for imaging breasts, the apparatuscomprising a breast compression plate for compressing a breast to beimaged, the breast compression plate having a vertical adjustment meansfor adjusting the height of the breast compression plate to a selectedheight, and an optically-readable pattern; a breast imaging means forimaging the breast compressed by the compression plate; an opticalmeasurement means for generating an image of the optically-readablepattern; and, calculating means for determining a deflection of thebreast compression plate from the selected height from the image of theoptically readable pattern.
 7. The mammography machine as defined inclaim 6 wherein the pattern has a plurality of local pattern indicia;the image has a plurality of local image indicia including an associatedlocal image indicia for each local pattern indicia in the plurality oflocal pattern indicia; and for each local pattern indicia in theplurality of local pattern indicia, the data processing means isoperable to determine an associated local deflection of the breastcompression plate from the associated local image indicia.
 8. The methodas defined in claim 6 wherein the pattern is a two-dimensionalradio-lucent pattern.
 9. The method as defined in claim 6 wherein thepattern is a grid, and each local pattern indicia is a point on thegrid.